1. Field of the Invention
The present invention relates to a measurement method and a measurement apparatus by a tracking type laser interferometer, and particularly to a measurement method and a measurement apparatus by a tracking type laser interferometer capable of expanding a measurement range and improving its measurement accuracy.
2. Description of the Related Art
As an apparatus that controls the emission direction of a laser beam being measurement light, tracks a recursive reflector (also called a retro-reflector) attached to a measurement object, and measures the distance to the recursive reflector by laser interference at high accuracy, a tracking type laser interferometer exists. Hereinafter, a description is given below of a tracking type laser interferometer represented by Japanese Patent Publication No. 2603429 (called Patent Document 1) based on a brief configurational view of FIG. 1.
The tracking type laser interferometer includes a retro-reflector 20 attached to a measurement object 70, means (not illustrated) for emitting measurement light, light receiving means 30, emission direction controlling means 40, and controlling means 50. And, the main body 10 is composed of the means (not illustrated) for emitting measurement light, the light receiving means 30 and the emission direction controlling means 40.
The retro-reflector 20 is an optical element in which the optical axis of incident light and that of emission light are parallel to each other, and the optical axis of incident light is symmetrical to the optical axis of reflecting light with respect to the center of the retro-reflector 20. Therefore, the retro-reflector 20 includes a function (the measurement light returned by reflection is called “return light”) of reflecting and returning incident measurement light in the incident direction. Also, where measurement light is made incident into a position differ from the center of the retro-reflector 20, the position of the optical axes of the measurement light and the return light differ from each other. Therefore, the shift amount of the optical axes will be observed by the second light receiving means 36 described later.
The means for emitting the measurement light emits a part of a laser beam toward the retro-reflector 20 by means of a half mirror and so on. In addition, the other part of the laser beam is made incident into the first light receiving means 31 described later, as reference light for distance measurement.
The light receiving means 30 is composed of first light receiving means 31 used for measuring the distance from the reference point 60 in the main body 10 to the retro-reflector 20 fixed in the measurement object 70, and the second light receiving means 36 used for controlling and tracking the displacement of the retro-reflector 20. The first light receiving means 31 receives return light reflected by the retro-reflector 20 and returning therefrom and the reference light described above, and transmits the light receiving signals to the controlling means 50. On the other hand, the second light receiving means 36 receives the optical axis of the measurement light and the optical axis of the return light, and transmits signals pertaining to the shift amount of the optical axes to the controlling means 50.
The emission direction controlling means 40 is composed of a turning mechanism of two axes orthogonal to each other. In this case, the center position that is common to the two axes is made into the reference point 60. In addition, the shift amount of both optical axes, which is transmitted from the second light receiving means 36, is kept constant by the emission direction controlling means 40 at all times, so that distance measurement made by the first light receiving means 31 is not discontinued.
The controlling means 50 obtains a distance in response to an increase or a decrease in the distance between the reference point 60 and the retro-reflector 20 based on signals transmitted from the first light receiving means 31. Also, the controlling means 50 controls the emission direction controlling means 40 based on signals transmitted from the second light receiving means 36 so as to turn the emission direction of the measurement light to the retro-reflector 20.
With such a structure and functions, the turning angle information of the emission direction controlling means 40 and the distance observed by the first light receiving means 31 or the distance observed by the first light receiving means 31 are outputted as measurement values of the tracking type laser interferometer. That is, if the tracking type laser interferometer is used for measurement of three-dimensional coordinate values, in the former case, it is possible to obtain the coordinate values in a three-dimensional space directly from the measurement values, and in the latter case, it becomes possible to obtain similar coordinate values by carrying out three-side length measurement with a plurality of tracking type laser interferometers combined.
However, the measurement range of a conventional tracking type laser interferometer represented by Patent Document 1 described above is restricted by an alterable range of emission direction of the emission direction controlling means 40 and a reflectable range of a retro-reflector 20 being a recursive reflector. And, since electric wires and optical fibers are connected to the tracking type laser interferometer 10, there is restriction resulting from the structure thereof. Therefore, a dead angle that cannot be measured may be brought about in the measurement space.
For example, when the turning angle is considered in the paper surface, if the alterable range of the emission direction of the emission direction controlling means 40 is the angular range 12 shown in FIG. 2, no light can be made incident into the retro-reflector 20 in the range other than the above, wherein the measurement range is restricted to the angular range 12.
Also, since, with the conventional tracking type laser interferometer, the distance from the reference point 60 being the center of turning of the emission direction controlling means 40 to the retro-reflector 20 is measured, a change in distance is an order of the cosine error when the retro-reflector 20 is displaced vertically with respect to the optical axes composed of the reference point 60 and the retro-reflector 20, it is hardly reflected in distance measurement.
For example, when a case where upward displacement δ occurs on the paper surface of FIG. 3 is taken into consideration, the distance between the reference point 60 and the retro-reflector 20 changes from L to L1. At this time, the relationship between L, L1 and δ is expressed by equation (1) below.L1=(L2+δ2)1/2=L*(1+(δ/L)2)1/2  (1)
Herein, if δ is small in comparison with L, Equation (1) becomes Equation (2) below.L1≈L*(1+(δ/L)2/2)≈L   (2)
The change in the distance measurement value, which results from the retro-reflector 20 being displaced by δ, is a secondary amount of a minute quantity (δ/L), and it is not reflected in the distance measurement amount. Thus, there exists a position and a direction (called a specific point) where the length measurement sensitivity is remarkably worsened.
Also, in order to solve these problems and shortcomings, it is considered that the installation position of a tracking type laser interferometer is changed. However, there are cases where the tracking type laser interferometer cannot be moved, for example, cases where any interfering object exists in a position where it is desired to be installed and where no appropriately stable installation place can be found. Furthermore, since the tracking type laser interferometer is a precision measurement device, there are problematic points in installation and adjustment in connection with movement thereof.